An abrasive article usually consists of an abrasive coating attached to a backing. During most abrading applications, the exposed surface of an abrasive coating contacts, under pressure, an exposed surface of a workpiece. The abrasive coating becomes altered by the contact and the movement of the abrasive article against the workpiece. Such contact and movement have been known to cause the removal of an abrasive coating from an abrasive article. Forceful surface modification processes in which an abrasive article, contacts a hard workpiece, under high pressure, have been known to quickly erode the abrasive coating from the backing of an abrasive article, rendering the abrasive article ineffective in subsequent abrading applications.
A durable abrasive article is capable of contacting a workpiece under pressure for a long duration, if necessary, or contacting the surface of a large number of individual workpieces, under pressure, for short durations, and yet adequately abrade or polish the surface of the workpiece(s). An abrasive article with a thick abrasive coating theoretically should have a greater durability than an abrasive article with a thin abrasive coating, because it should take longer for the surface modification process to wear away the additional abrasive coating of the thick abrasive article.
Typically, abrasive coatings of lapping coated abrasive articles, or fixed abrasive articles, comprise a single layer of abrasive material. The thickness of these single layer abrasive coatings may vary in size depending upon the components that make up the abrasive coating. For example, the thickness of an abrasive coating of a lapping coated abrasive article maybe 7 .mu.m, 10 .mu.m, or 22 .mu.m. These abrasive coatings are typically formed by applying an abrasive material containing abrasive particles dispersed in a binder to a backing. The binder typically contains either radiation or thermally curable precursor polymer subunits and initially flows when applied to a backing. Then, the precursor polymer subunits of the abrasive coating are cured by the application of heat and/or radiation and the curable abrasive coating is converted into a hard, or cured, abrasive coating.
Sometimes problems develop during the curing of a curable abrasive coating. The physical characteristics of a particular abrasive coating (its components) and/or a particular curing process will affect the outcome of the manufactured abrasive article. One curing process utilizes abrasive coatings containing radiation curable precursor polymer subunits and a radiation energy source. Radiation energy applied to an abrasive coating should be able to initiate polymerization reactions, or crosslinking reactions, so that the precursor polymer subunits become part of a larger polymer chain. In addition, the radiation energy, in part, must also be able to penetrate the abrasive coating so that underlying material becomes essentially cured. Specifically, radiation energy is typically only able to polymerize or crosslink suitable precursor polymer subunits in radiation penetrable regions of the abrasive coating. In regions of the abrasive coating not penetrable by radiation, the precursor polymer subunits do not readily become part of a larger polymer chain through chemical reactions.
Typically, there is a practical limit as to the thickness a specific type of radiation energy is able to penetrate a curable abrasive coating. Factors related to the penetration of radiation through an abrasive coating and the practical thickness limit of an abrasive coating include, but are not limited to, the amount and type of precursor polymer subunits in a curable abrasive coating, the amount and type of abrasive particles in a curable abrasive coating, the length of time the radiation energy is applied to a curable abrasive coating, the type of radiation energy applied to a curable abrasive coating, and the type of photoinitiator present in a curable abrasive coating. If the thickness of the abrasive coating is greater than its practical thickness limit, the region of the abrasive coating beyond this practical thickness limit is not penetrable by the applied radiation. An abrasive coating having undergone a radiation curing process where its thickness of the abrasive coating is greater than its practical thickness limit may only partially cure. A partially cured abrasive article used in a surface modification process will tend to result in the surface modification process quickly wearing the abrasive coating from the backing of the abrasive article. This abrasive article with a partially cured coating may not properly abrade or polish a workpiece if continued to be used in the same surface modification process, or if the abrasive article is used in future modification processes in combination with new workpieces.
Alternatively, an abrasive coating may contain thermally curable precursor polymer subunits, instead of radiation curable precursor polymer subunits. Heat is able to penetrate most abrasive coating compositions, if the abrasive coating is heated long enough. However, as the thickness of an abrasive coating increases, heat tends to crack the abrasive coating during the heat curing step during the manufacture of the abrasive article. There is a practical thickness limit inherent in abrasive coatings that are heat curable that can be defined as the thickness of the abrasive coating at which cracking tends to occur upon heat curing. This practical thickness limit would be dependent on factors such as the components of the abrasive coating, the duration of the heat curing step, and the temperature of the thermal curing process. It is not unusual that heat curing of very thick layers of abrasive coating may result in voids and air bubbles being formed in the coating during thermal curing of the coating. Abrasive articles, containing cracked or bubbled abrasive coatings may result in non-uniform abrading or polishing and are likely to wear quickly when used in surface modification processes.